Interference Randomization of Control Channel Elements

ABSTRACT

A method and a radio base station for interleaving control channel data to be transmitted in a telecommunications system are described. The method comprises grouping the control channel elements CCE 1 -CCEn into a first order of control channel symbol groups, adding symbol groups comprising dummy values or zeros to the first order of control channel symbol groups based on a number of available symbol group positions, interleaving the first order of the control channel symbol groups resulting in an a second order, and mapping the second order of control channel symbol groups to the control channel transmission resources.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/679,419, which was the National Stage of International ApplicationNo. PCT/SE2008/050372, filed Mar. 31, 2008, which claims the benefit ofU.S. Provisional Application 60/974,949, filed Sep. 25, 2007, thedisclosures of each of which are incorporated herein by reference intheir entirety.

TECHNICAL FIELD

The present invention relates to a method and arrangement in atelecommunication system, in particular it relates to a method andarrangement for symbol group interleaving in a telecommunication system.The present invention also relates to a method for mapping symbol groupsto be interleaved in a telecommunication system.

BACKGROUND

Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) isthe name given to a project in 3GPP to improve the UMTS mobile phonestandard to future requirements. Orthogonal Frequency-DivisionMultiplexing (OFDM) is a digital multi-carrier modulation scheme that isused in LTE. The structure of the OFDM signal in the LTE containsresource elements spaced in time, so-called OFDM symbols, and frequency,so-called OFDM sub-carriers. These resource elements are grouped into acollection of resource blocks that make up the OFDM signal to betransmitted. Within this collection of resource blocks, certain resourceelements are designated to contain the control channel signalinginformation, and base stations within each cell must transmit thesecontrol channel resource elements to the various mobiles i.e. mobileterminals, also called user equipment (UE) in LTE, contained withinthose cells. The transmissions from different cells potentially overlapin either time or frequency and may interfere with each other.

Additionally, techniques such as power control may be used for controlchannel signaling. This affects the level of interference that affects amobile and it may create an uneven distribution of interference todifferent mobiles. If certain control channel elements from one basestation is transmitted with high power, they might cause disturbance tocorresponding control channel elements transmitted from another basestation. One technique to overcome this uneven interference scenario isto use interference avoidance, where transmissions are coordinatedbetween base stations, also called NodeBs or eNodeBs in LTE, so that areduced level of interference is attained at the mobiles.

Alternatively, techniques can be used for making interference appearrandomly, which results in that no mobile experiences the sameinterference pattern repeatedly. In the LTE system, an interferencerandomization technique is proposed to be used for control channelsignaling.

In the suggested approach the control channel elements are interleavedand mapped to LTE transmission resource elements so that there is arandomization of interference between control channels from differentcells. A control channel element (CCE) is the control channelinformation to one or more mobiles. A control channel element group (CCEgroup) is provided as a concatenation of control channel elements,possibly with a different power level set for each CCE. The CCE group isthen mapped to a set of pre-defined control channel transmissionresource elements and transmitted. Currently proposed approaches musthave a common interleaving scheme for CCE groups transmitted fromdifferent cells followed by a cell-specific cyclic shift in order toreduce the number of information symbols transmitted from different CCEsthat share common OFDM transmission resource elements, i.e. which createinterference between the CCEs. This occurs prior to the mapping totransmission resource elements. The cyclic shift parameter may be tiedto the cell identity (ID), for example, so the mobile can easily obtainthe cyclic shift parameter. Different interleaving schemes may be used.

Consider the interleaving scheme in an LTE transmission. Let the systemhave a bandwidth of 5 MHz, so that 24 resource blocks are available foruse (note, one resource block consists of 12 OFDM subcarriers spanningover the horizontal dimension and 7 OFDM symbols that span over thevertical dimension as illustrated by the blocks in FIGS. 1, 2, 4 and 5.Let there be two transmit antennas and one to three OFDM symbols usedfor the CCE group. Interleaving is done in groups of four OFDM tilesi.e. across four adjacent or nearly adjacent resource elements infrequency, so that space-frequency block coding is permitted. Each groupof four resource elements is called a symbol group. Symbol groups canalternatively be designed consisting of more or fewer resource elements.

In a configuration where the first three OFDM symbols are used forcontrol channel signaling, there are eight symbol groups per eachindividual resource block and 192 symbol groups in total across the 24resource blocks covering the 5 MHz bandwidth.

As an example, FIG. 1 shows the symbol groups (8, 9 and 10) located inone resource block 5 consisting of 12 subcarriers. Only the first threeOFDM symbols 7 are shown as these three are potentially used for controlchannel signaling. Groups of four OFDM tiles make up symbol groups (8, 9and 10) e.g. the tiles labeled 1 form the first symbol group. Thestriped and checkered tiles 6 correspond to reference tiles used, forexample, for channel estimation and are not available for control ordata channel transmission.

A CCE group consisting of 72 symbol groups has been considered for a 5MHz bandwidth. This might correspond to using all the resource elementsin one OFDM symbol, provided that there are no pilot tiles located inthat symbol. This corresponds to FIG. 1. The performance of two symbolinterleaving patterns has been considered. The patterns are a prunedbit-reversal interleaver. The same interleaver structure is used in allcells and interference randomization is accomplished via a cell-specificcyclic shift of the interleaved pattern prior to the mapping to resourceelements.

Within one CCE group, a number of control channel elements areconcatenated and transmitted. In the example illustrated in FIG. 1consisting of 72 symbol groups, there are 9 CCEs each consisting ofeight symbol groups.

FIG. 2 shows the concatenation of symbols groups 22 contained withinCCE1 20 through the symbol groups 23 contained within CCE9 24 in acontrol channel element group 21. The eight symbol groups 22 making upCCE1 20 are marked with value 1, while those making up CCE 2 25 and CCE924 are marked with value 2 and 9 respectively. In a different cell, ifthe CCE group to be transmitted has the same format, then twotransmissions interfere when the symbol groups from the two CCEs collidewith one another. By measuring the number of collisions, it is possibleto determine the collision rate performance of the various approaches.Using the interleaving and cyclic shift operations, the amount ofinterference (i.e. the number of collisions) is potentially reduced.This process is shown in FIG. 3, where the control channel elementgroups 30 first are grouped together in step 31. The control channelelements are then interleaved in step 32. In step 33 a cell-specificcyclic shift is applied to the interleaved control channel elementgroups.

The performance for the two interleaving patterns from R1-072225, “CCEto RE mapping”, RAN1#49, Kobe, Japan, May 2007 and R1-072904, “CCE to REinterleaver design criteria”, RAN1#49bis, Orlando, USA, June 2007 isshown in FIG. 10. Performance is evaluated by cyclically shifting theinterleaved CCE group and then finding the number of overlapping controlchannel elements with the same control channel element number, and isthe same approach used to evaluate the results in R1-072904, “CCE to REinterleaver design criteria”, RAN1#49bis, Orlando, USA, June 2007. FromFIG. 10, the pruned bit-reversal interleaver (PBRI) pattern has highpeak correlations when the CCE is shifted by a multiple of nine symbolgroups, which the new pattern in R1-072904, “CCE to RE interleaverdesign criteria”, RAN1#49bis, Orlando, USA, June 2007 avoids these peaksfor non-zero shifts.

Consider next a uniformly random interleaving pattern in place of eitherthe two approaches considered in R1-072904, “CCE to RE interleaverdesign criteria”, RAN1#49bis, Orlando, USA, June 2007. This implies arandom permutation of symbol groups before the cyclic shift. To get anidea of the performance under this truly random symbol permutationapproach, the mean collision rate for 200 random realizations is shownin FIG. 11. Of course, not all random realization will have adequatefrequency diversity, compared to the approaches used in R1-072904, “CCEto RE interleaver design criteria”, RAN1#49bis, Orlando, USA, June 2007.While randomization of the interfering CCE groups reduces interferenceconsistently, a truly random symbol permutation is not practical due tothe required signaling aspects between the base station and the mobilefor such a scheme.

The difficulty in using the approaches considered in R1-072904, “CCE toRE interleaver design criteria”, RAN1#49bis, Orlando, USA, June 2007 isthat they are defined for a specific number of symbol groups mapped tothe CCE size and/or the CCE group size. When also accounting forfrequency diversity, these approaches are also defined for specificfrequency bandwidths. When these parameters change, the interleavingpatterns are either no longer valid or may not satisfy the designrequirements, i.e. they are not flexible to changing CCE or CCE groupsizes, or bandwidth or OFDM symbol allocations in the control channeltransmission resources. Consequently, a more flexible approach whoseperformance approaches the performance of the random realization schemeshown previously is preferred.

SUMMARY

The present invention aims at providing a solution that alleviates atleast one of the problems indicated above.

The present invention intends to provide a common approach for symbolgroup permutation, i.e. symbol group interleaving, that accomplishes atleast one of the following objectives:

-   -   providing a different symbol group interleaving pattern to be        used in different cells;    -   flexibility in that it can handle a different frequency        bandwidth in the control channel transmission resources, a        different number of OFDM symbols in the control channel        transmission resources, and a different number of symbol groups        in the information stream;    -   providing interference randomization performance comparable to        that of the random interleaver discussed above in the average        sense; and    -   providing frequency diversity as well as interference        randomization.

A solution according to the present invention for accomplishing theabove objectives is to use a flexible symbol group interleaver that canhandle control channel parameters that may not be fixed in time or inthe cellular system. These parameters include: the number of OFDMsub-carriers in the control channel transmission resources; the numberof OFDM symbols in the control channel transmission resources; thenumber of symbol groups in the control channel signal; and the number ofavailable symbols groups in the control channel transmission resourcesfor placing the control channel symbol groups. Combined with an optionalcell-specific cyclic shift, this allows for interference randomization.Further, by regrouping symbol groups by resource block order in thecontrol channel transmission resources and designing the symbol grouppermutation pattern taking the resource block size into account,frequency diversity can be provided. An additional aspect of thisapproach is that interference can be further reduced by advantageouslylocating the control channel symbol groups within the available OFDMsymbols in the control channel transmission resources in case there is agreater number of potential symbol groups available in the controlchannel transmission resources than are required within the controlchannel information stream.

At least one of the above objects is achieved with a method orarrangement according to the appended independent claims. Furtherobjects and advantages are evident from the dependent claims.

A first aspect of the present invention relates to a method forrandomization of interference experienced by a shared control channeltransmitted by using control channel transmission resources from a basestation. The shared control channel comprises control channel elementsCCE1-CCEn. The control channel elements CCE1-CCEn are grouped into afirst order of control channel symbol groups and then determining anumber of available symbol group positions of the control channeltransmission resources. Symbol groups comprising dummy values or zerosare added to the first order of control channel symbol groups so thatthe first order of symbol groups substantially equals the number ofavailable symbol group positions. Interleaving the first order of thecontrol channel symbol groups resulting in a second order for thecontrol channel symbol groups. The second order of control channelsymbol groups is mapped to the control channel transmission symbol grouppositions.

A second aspect of the present invention relates to a radio base stationconfigured to randomization of interference experienced by a sharedcontrol channel, where the shared control channel is transmitted byusing control channel transmission resources from the base station. Theshared control channel comprises control channel elements CCE1-CCEn andthe base station comprises a processing circuit for grouping the controlchannel elements CCE1-CCEn into a first order of control channel symbolgroups. A processing circuit is also provided for adding symbol groupscomprising dummy values or zeros to the first order of control channelsymbol groups so that the first order of symbol groups substantiallyequals a number of available symbol group positions. Further, aprocessing circuit interleaves the first order of the control channelsymbol groups resulting in a second order for the control channel symbolgroups. The radio base station also has a processing circuit for mappingthe second order of control channel symbol groups to the control channeltransmission symbol group positions.

The described aspects and embodiments of the invention provide theadvantage of renumbering of the symbol groups so that frequencydiversity can easily be accounted for in the interleaver design.

Another advantage is that all available symbol groups are used, not onlythose contained in the control channel information.

Yet another advantage is enhanced performance due to better mapping orplacing of the used symbol groups from a larger subset of symbolsgroups.

A further advantage is flexibility with respect to the frequencybandwidth, number of OFDM symbols, and number of information symbolgroups used for control channel signaling.

In the following, preferred embodiments of the invention will bedescribed with reference to the enclosed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates symbol groups defined over one OFDMsymbol according to prior art.

FIG. 2 in turn schematically illustrates a control channel element groupusing 72 symbol groups according to prior art.

FIG. 3 illustrates an example of prior art CCE group interleaving withcyclic shift.

FIG. 4 illustrates symbol groups defined over two OFDM symbols.

FIG. 5 illustrates symbol groups defined over three OFDM symbols.

FIG. 6 shows a first embodiment of the invention.

FIG. 7 illustrates a second embodiment of the invention using a QPPsymbol group interleaver.

FIG. 8 illustrates a second preferred embodiment of the invention withcell specific interleaver.

FIG. 9 illustrates an example of cyclic sampling for 72 symbol groups.

FIG. 10 shows symbol group collision rate for interleaving approachesaccording to prior art in R1-072904, “CCE to RE interleaver designcriteria,” Huawei, RAN1#49bis, Orlando, USA, June 2007.

FIG. 11 illustrates symbol group mean collision rate for randominterleavers.

FIG. 12 illustrates collision rate for embodiment 1 of the presentinvention using QPP interleaver.

FIG. 13 shows mean collision rate for embodiment 2 of the inventionacross different prime number interleavers.

FIG. 14 illustrates symbol groups defined over three OFDM symbols andordered by OFDM symbol within a resource block.

FIG. 15 schematically illustrates a base station according to anembodiment of the invention.

DETAILED DESCRIPTION

In the following, various embodiments of the invention will bedescribed. The approach described below is intended for flexibility withperformance approaching that of the random interleaving approachdescribed above. Considering the 72 symbol group example discussedabove, where the cell-specific cyclic shift is intended to randomizeinterference between cells. However, when changing CCE or CCE groupsizes, or bandwidth or OFDM symbol allocations in the control channeltransmission resources, it becomes difficult to design an interleavingpattern to handle this variability. Consider the case of having 72symbol groups within a CCE transmitted within a 5 MHz bandwidth. Thereare only 48 sub-carriers available in the first OFDM symbol, thereforeeither two or three OFDM symbols must be used to transmit the entireCCE. FIG. 4 shows the two-symbol structure with resource block 40 whileFIG. 5 shows the three symbol structure of the control channeltransmission resources with resource block 50. One new aspect accordingto the present invention shown in FIGS. 4 and 5 is that the symbolgroups are numbered sequentially within a resource block 40 (FIG. 4) and50 (FIG. 5) rather than across OFDM sub-carriers. When combined withsymbol group interleaving, the numerical distance between symbol groupsnow can be used as a measure of frequency diversity. For example, usingthe two OFDM symbols as in FIG. 4, when the distance between two symbolgroups is greater than 5, then these two symbol groups reside indifferent resource blocks. A distance of 8 symbol groups would imply thesame for the control channel configuration shown in FIG. 5.

It should be clear to those skilled in the art that the numberingmethodology taught herein is to ensure the numbering provides a goodrepresentation of frequency separation. Hence, numbering methodsaccording to the present invention are not limited to those shown inFIG. 4 and FIG. 5. The sequential numbering can be mapping sequentiallyover OFDM symbols within the symbol group positions followed by mappingsequentially over frequencies within said symbol group positions, asshown in FIG. 14. In FIG. 14 reference number 140 denotes a resourceblock. Therefore, a distance of 8 symbol groups results in that symbolgroups reside in different resource blocks.

A second aspect disclosed by this invention is that while there are 72symbol groups contained within the control channel information, thereare 120 available symbol group positions in the control channeltransmission resources within the first two OFDM symbols where the CCEmay be inserted (192 in the first three OFDM symbols). This canadvantageously be used to further randomize interference and results ineven less interference between control channels from different cells.

However, note that not all symbol group positions may be available forpotential randomization purposes. In fact, some of the symbol grouppositions may be fixed for other fields, such as the Physical Hybrid-ARQIndicator Channel (PHICH) and Physical Control Format Indicator Channel(PCFICH) portions of the control channel. This implies that fewer than120 symbol groups will be available when using two OFDM symbols (or 192when using three OFDM symbols). For example, if 4 symbol groups arededicated to the PCFICH field and 3 symbol groups dedicated to the PHICHfield, then there are 113 symbol groups available for interferencerandomization. This demonstrates why flexibility is important indesigning the interleaver design.

Next, consider the 72 symbol group CCE to be transmitted in the 113possible symbol groups available over the 5 MHz bandwidth controlchannel transmission resources in the first two OFDM symbols. A similarapproach of interleaving plus cyclic shifting can be used forinterference randomization. However, now let the symbol groups in thecontrol channel information be ordered from 1 to 113 using the resourceblock order described in this section, where the last 41 symbol groupsconsist of “dummy values” or zeros, i.e. groups that are not transmittedafter symbol permutation and cyclic shifting. This not only randomizesthe positions of the resulting symbol groups, but also avoidsinterference from some symbol groups in the control channel transmissionresources from other cells that do not occupy the same channeltransmission resource elements. One aspect of this approach is that thepreviously considered interleaver designs are no longer applicable inthis new format, since the interleaver (i.e. the symbol grouppermutation) must work with a length of 113 in this example, and thisnumber may change depending on the number of OFDM symbols used forcontrol channel signaling or the number of PHICH and PCFICH symbolgroups to be transmitted. Below we consider two embodiments of thepresent invention that incorporate the above two aspects together withflexible interleaver designs that randomize interference.

A first embodiment is shown in FIG. 6, by illustrated flowchart. In themethod, control channel elements CCE1-CCEn are grouped into a firstorder of control channel symbol groups in step 601. In the next step 602a number of available symbol group positions of control channeltransmission resources are determined. The number of available symbolgroup positions in step 602 may e.g. be based on a number of OFDMsymbols and OFDM subcarriers used for the control channel transmissionresources. Based on the number of symbol groups that are available inthe control channel transmission resources, symbol groups comprising“dummy” values or zeros are added in step 603 to the first order ofcontrol channel symbol groups so that the first order of symbol groupssubstantially equals the number of available symbol group positions inthe control channel transmission resources. The first order of thecontrol channel symbol groups are then interleaved in step 604 resultingin a second order for the control channel symbol groups. A cyclicshifting of the second order is then followed in step 605 before thecyclic shifted second order of control channel symbol groups are mappedto the control channel transmission symbol group positions in step 606.The cell-specific cyclic shift in step 605 may for example be determinedbased on the cell ID. The mapping of control channel symbol groups instep 606 may first be done sequentially over frequencies within thesymbol group positions followed by a sequentially mapping over OFDMsymbols within the symbol group positions or the other way around, whichmeans that the mapping in step 606 first is done sequentially over OFDMsymbols within the symbol group positions followed by a sequentiallymapping over frequencies within the symbol group positions.

Alternatively the mapping in step 606 may be done sequentially withrespect to symbol groups within resource blocks of said control channeltransmission resources. The mapping can then first be done sequentiallyover frequencies within the resource block followed by a sequentiallymapping over OFDM symbols within the resource block, or the other wayaround, which means that the mapping in step 606 first is donesequentially over OFDM symbols within the resource block followed by asequentially mapping over frequencies within said resource block.

To achieve flexibility, the Quadratic Permutation Polynomials (QPP)based interleaver design may be used for interleaving in step 604, as itcan handle a varying length of symbol groups. Interleavers based on thequadratic permutation polynomials were proposed and designed to be usedin a turbo code J. Sun and O. Y. Takeshita, “Interleavers for turbocodes using permutation polynomials over integer rings,” IEEE Trans.Inform. Theory, vol. 51, no. 1, pp. 101-119, January 2005. For instanceis a table of all 188 different QPP interleaver parameter sets definedfor LTE turbo code 3G Partnership Program in Technical Specifications36.212 v8, “Multiplexing and Channel Coding (Release 8),” 2007.

Each QPP interleaver is defined by three parameters: length K and thepolynomial coefficients f₁ and f₂. The relationship between the outputindex i and the input index Π(i) satisfies the following quadratic form:

π(i)=(f ₁ ×i+f ₂ ×i ²)mod K

For instance, the interleaving addresses or address values for K=40 (andf₁=3, f₂=10) are

0 13 6 19 12 25 18 31 24 37 30 3 36 9 2 15 8 21 14 27 20 33 26 39 32 538 11 4 17 10 23 16 29 22 35 28 1 34 7

According to another embodiment of the invention the step 604 ofinterleaving, in the method according to FIG. 6, may further comprisethe steps as illustrated in FIG. 7.

Let N_(SG) represent the number of available symbol group positions inthe control channel transmission resources determined in step 602 ofFIG. 6. Find in step 701 (FIG. 7) the minimum value of parameter K in apredefined look up table e.g. in Table 1 such that K≦N_(SG) and selectin step 702 a further set of parameters (f₁, f₂, i) based on the value Kin the look-up table. Compute the interleaving addresses of the selectedQPP interleaver in step 703. If K>N_(SG), the out-of-bound addresses(i.e., those higher than N^(SG)−1) are truncated.

For instance, if N_(SG)=35, then the QPP with K=40 (and f₁=3, f₂=10)will be selected. After truncation, the interleaving addresses for thepadded CCE group are

0 13 6 19 12 25 18 31 24 30 3 9 2 15 8 21 14 27 20 33 26 32 5 11 4 17 1023 16 29 22 28 1 34 7

According to another embodiment of the present invention yet anotherinterleaving approach may be used in step 604.

This different approach is shown in FIG. 8. Rather than to use the sameinterleaver, i.e. symbol group permutation, in each cell, acell-specific interleaver is used in step 805. Note, in the methodaccording to FIG. 7 a different interleaver would be used when there area different number of available symbol groups present in the controlchannel transmission resources in each cell. However, in the case whenthe number of available symbol groups in the control channeltransmission resources is the same in each cell the method in FIG. 8explicitly uses a different interleaver design in step 805 to furtherrandomize interference. In the approach according to FIG. 8, theinterleaver design in step 805 is a linear interleaver using a specificprime number P. The value of P is cell-specific and may, for example, bechosen based on a prime-number lookup table 803 based on the cellidentity. The method according to FIG. 8 is described in more detailbelow.

Consider a permutation pattern for the numbers in natural order 1through 72 that represents the 72 symbol groups in the control channelsignal. In step 805 let the permutation be a cyclic sampling of thesenumbers by a prime number P that is not a factor of 72. For example,consider the cyclic sampling of the numbers 1 through 72 by a samplespacing of P=7, starting with the number 1. The result is a sequence ofthe original numbers 1 through 72 now arranged according to FIG. 9. Toensure that the interleaver is cell specific, let both the samplespacing in step 805 of the symbol groups and the chosen cyclic shift bedetermined by the cell ID. In order to accommodate many cell specificsymbol group permutation patterns, a procedure for selecting theallowable sample spacing values is defined in step 805. Let the orderedset of prime numbers from one to some large N (with some restrictionsdescribed below) be used as these values. Let P be the cyclic samplespacing and N_(SG) be the number of symbol group positions in thecontrol channel transmission resources, the restrictions on choosingprime numbers can be set as follows:

Disallow a value P that is a factor of N_(SG);

To enable frequency diversity, only allow values of P that fall in therange δ≦mod(P−1, N_(SG))+1≦N_(SG)−δ where δ is some integer greater than2. For example, setting δ=8 for the approach shown in FIG. 5 placesadjacent symbol groups in different resource blocks. The performance ofthis approach is shown in FIG. 13, comparing the collisions betweendifferent cells with different values of P. Note, since 120 availablesymbols in the control channel transmission resources are assumed inthis example, the mean collision rate is lower because of the use of thezero-padded, dummy symbol groups.

Some considerations for the above described preferred embodiments are:The cell-specific cyclic shift operations may be performed prior to theinterleaving function i.e. their order may be swapped. In the methodaccording to FIG. 6 step 605 may thus be performed before step 604. Thenull symbol groups or symbol groups comprising “dummy” values may becombined with the used symbol groups in other ways than appending thenull symbol groups. They may rather be pre-pended, for example, orotherwise combined with the used symbol groups. Instead of “dummyvalues” other data than control channel data can be put in the symbolgroups that are not used for control channel data. Other data thancontrol channel data can also be mixed with “dummy values” and/or zerosand put in the symbol groups that are not used for control channel data.

The above described methods may be applied in radio base stationssupporting e.g. LTE.

Turning now to FIG. 15 illustrating schematically a radio base stationaccording to embodiments of the present invention. The radio basestation 150 comprises means 151 for grouping control channel elementsCCE1-CCEn into a first order of control channel symbol groups. The radiobase station comprises further means 152 for determining a number ofavailable symbol group positions of control channel transmissionresources. Means 153 are also provided for adding symbol groupscomprising “dummy” values or zeros to the first order of control channelsymbol groups so that the first order of symbol groups substantiallyequals the number of available symbol group positions in control channeltransmission resources. The base station comprises further means 154 forinterleaving the first order of the control channel symbol groupsresulting in a second order for the control channel symbol groups. Means155 are also provided for cyclic shifting the second order and there arefurther means 156 for mapping the cyclic shifted second order of controlchannel symbol groups to the control channel transmission symbol grouppositions.

The approaches for interference randomization that are disclosed havethe flexibility with respect to the frequency bandwidth, number of OFDMsymbols, and number of information symbol groups used for controlchannel signaling. The approach can, of course, be applied to othersituations where interference randomization is required. Any examplesand terminology relating to 3GPP LTE standard being used herein shouldnot be seen as limiting the scope of the invention, the methodology ofwhich in principle can be applied to any communication system usingsymbol interleaving. Means mentioned in the present description can besoftware means, hardware means or a combination of both. The describedsubject matter is of course not limited to the above described and inthe drawings shown embodiments, but can be modified within the scope ofthe enclosed claims.

What is claimed is:
 1. (canceled)
 2. A method for randomization ofinterference experienced by a shared control channel transmitted byusing control channel transmission resources from a base station wherethe shared control channel comprises control channel elements CCE1-CCEn,comprising: grouping the control channel elements CCE1-CCEn into a firstorder of control channel symbol groups; adding symbol groups comprisingdummy values or zeros to the first order of control channel symbolgroups so that the first order of symbol groups substantially equals anumber of available symbol group positions; interleaving the first orderof the control channel symbol groups resulting in a second order of thecontrol channel symbol groups; and mapping the second order of controlchannel symbol groups to the control channel transmission symbol grouppositions.
 3. The method of claim 2: wherein the transmission resourcescomprise a number of Orthogonal Frequency Division Multiplexing (OFDM)symbols and OFDM subcarriers; and wherein the number of available symbolgroup positions of the control channel transmission resources aredetermined based on a number of OFDM symbols and OFDM subcarriers usedfor the control channel transmission resources.
 4. The method of claim2, wherein the mapping comprises mapping sequentially over frequencieswithin the symbol group positions followed by mapping sequentially overOFDM symbols within the symbol group positions.
 5. The method of claim2, wherein the mapping comprises mapping sequentially over OFDM symbolswithin the symbol group positions followed by mapping sequentially overfrequencies within the symbol group positions.
 6. The method of claim 2,wherein the mapping comprises mapping sequentially with respect tosymbol groups within resource blocks of the control channel transmissionresources.
 7. The method of claim 6, wherein the mapping comprisesmapping sequentially over frequencies within the resource block followedby mapping sequentially over OFDM symbols within the resource block. 8.The method of claim 6, wherein the mapping comprises mappingsequentially over OFDM symbols within the resource block followed bymapping sequentially over frequencies within the resource block.
 9. Themethod of claim 2, wherein the interleaving is Quadratic PermutationPolynomials (QPP) based interleaving.
 10. The method of claim 9, whereinthe QPP based interleaving comprises: establishing the minimum value ofa parameter K in a predefined look-up table such that K≧N_(SG) whereN_(SG) is the number of available symbol group positions of the controlchannel transmission resources; selecting a further set of parameters(f₁, f₂, i) based on the value K in the look-up table; and computinginterleaving address values of the thus selected QPP interleaver byapplying the parameter values (K, f₁, f₂, i) in the formula:π(i)=(f ₁ ×i+f ₂ ×i ²)mod K where: K=length, f₁ and f₂=polynomialcoefficients, i=output index, and π(i)=input index.
 11. The method ofclaim 10, wherein the computing further comprises, if K>N_(SG),truncating any resulting interleaving address values higher than N_(SG)⁻¹.
 12. The method of claim 2, wherein the interleaving is cell-specificlinear interleaving.
 13. The method of claim 12, wherein thecell-specific linear interleaving is computed with a cell-specific primenumber P.
 14. The method of claim 13, wherein the interleaving is acyclic sampling of the first order by the cell specific prime number P,where the cell-specific primer number P is not a factor of the number ofsymbol group positions
 15. The method of claim 13, wherein thecell-specific prime number chosen is limited to prime numbers thatprovide sufficient frequency diversity as determined by a numericaldistance value between symbol groups that is above a certain thresholdvalue.
 16. The method of claim 2, wherein the adding comprises:appending or prepending and/or otherwise combining symbol groupscomprising dummy values or zeros to the first order of control channelsymbol groups.
 17. A radio base station configured for randomization ofinterference experienced by a shared control channel transmitted byusing control channel transmission resources from the base station,where the shared control channel comprises control channel elementsCE1-CCEn, and where the base station comprises: a processing circuitconfigured to group the control channel elements CCE1-CCEn into a firstorder of control channel symbol groups; a processing circuit configuredto add symbol groups comprising dummy values or zeros to the first orderof control channel symbol groups so that the first order of symbolgroups substantially equals a number of available symbol grouppositions; a processing circuit configured to interleave the first orderof the control channel symbol groups resulting in an a second order forthe control channel symbol groups; and a processing circuit configuredto map the second order of control channel symbol groups to the controlchannel transmission symbol group positions.
 18. The base station ofclaim 17, wherein transmission resources comprise a number of OrthogonalFrequency Division Multiplexing (OFDM) symbols and OFDM subcarriers andwherein the determining is based on a number of OFDM symbols and OFDMsubcarriers used for the control channel transmission resources.
 19. Thebase station of claim 17, wherein the processing circuit configured tomap is further is configured to: map sequentially over frequencieswithin the symbol group positions; and then map sequentially over OFDMsymbols within the symbol group positions.
 20. The base station of claim17, wherein the processing circuit configured to map is furtherconfigured to: map sequentially over OFDM symbols within the symbolgroup positions; and then map sequentially over frequencies within thesymbol group positions.
 21. The base station of claim 17, wherein theprocessing circuit configured to map is further configured to: mapsequentially with respect to symbol groups within resource blocks of thecontrol channel transmission resources.
 22. The base station of claim21, wherein the processing circuit configured to map is further isconfigured to: map sequentially over frequencies within the resourceblock; and then map sequentially over OFDM symbols within the resourceblock.
 23. The base station of claim 21, wherein the processing circuitconfigured to map is further configured to: map sequentially over OFDMsymbols within the resource block and then map sequentially overfrequencies within the resource block.
 24. The base station of claim 17,wherein the interleaving is Quadratic Permutation Polynomials (QPP)based interleaving.
 25. The base station of claim 24, wherein theprocessing circuit configured to interleave is further configured to:establish the minimum value of a parameter K in a predefined look-uptable such that K≧N_(SG) where N_(SG) is the number of available symbolgroup positions of the control channel transmission resources; select afurther set of parameters (f₁, f₂, i) based on the value K in thelook-up table; and compute interleaving address values of the thusselected QPP interleaver by applying the parameter values (K, f₁, f₂, i)in formula:π(i)=π(f ₁ ×i+f ₂ ×i ²)mod K where: K=length, f₁ and f₂=polynomialcoefficients, i=output index, and π(i)=input index.
 26. The base stationof claim 25, wherein the processing circuit configured to interleave isfurther configured to, if K>N_(SG), truncate any resulting interleavingaddress values higher than N_(SG) ⁻¹.
 27. The base station of claim 17,wherein the interleaving is cell-specific linear interleaving.
 28. Thebase station of claim 27, wherein the cell-specific linear interleavingis computed with a cell-specific prime number P.
 29. The base station ofclaim 28, wherein the cell-specific linear interleaving is a cyclicsampling of the first order by the cell specific prime number P, wherethe cell specific primer number P is not a factor of the number ofsymbol group positions.
 30. The base station of claim 28, wherein thecell-specific prime number chosen is limited to prime numbers thatprovides sufficient frequency diversity as determined by a numericaldistance value between symbol groups that is above a certain thresholdvalue.
 31. The base station of claim 17, wherein the processing circuitconfigured to add is further configured to: append or prepend and/orotherwise combine symbol groups comprising dummy values or zeros to thefirst order of control channel symbol groups.